The United States could sanction more banks with links to Russia and will step up enforcement against any dodging existing rules, a senior U.S. official said, as Western powers seek to reinvigorate efforts to isolate Moscow.

The comments come as Western diplomats seek to agree a new raft of sanctions for the anniversary of Russia's invasion of Ukraine on Feb. 24.

"We have immobilized about 80% of the assets in the Russian banking sector," said James O'Brien, head of the U.S. State Department's Office of Sanctions Coordination.